REFLECTION OF PHONONS IN 4He BY NaF

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Submitted on 1 Jan 1978

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REFLECTION OF PHONONS IN 4He BY NaF

N. Lockerbie

To cite this version:

JOURNAL DE PHYSIQUE Colloque C6, supplement au n" 8, Tome 39, aout 1978, page C6-246

REFLECTION OF PHONONS IN 4He BY NaF

N.A. Lockerbie

Service des Basses TempSratures, CEN Grenoble, 85 X, 38041 Grenoble Cedex, France

4 Résumé.- On a mesuré la distribution angulaire après réflexion sur une interface clivée NaF- He liquide, d'un faisceau collimaté venant de l'hélium sous un angle d'incidence de i< 10°. Les ré-sultats suggèrent l'existence d'états de l'interface qui n'interagissent qu'avec une partie du spectre de phonons incident.

Abstract.- The angular distribution after reflection at a cleaved NaF-liquid He interface of a collimated phonon beam incident from the helium at a grazing angle of incidence of ^ 10° has been measured. The results suggest the existence of interface states which interact with only a part of the incident phonon spectrum.

In the light of experiments which have been aarried out on the problem of phonon transmission across solid - He interfaces(Wyatt, Sherlock et al./l,2/, it is clear that for phonons which are

4

emitted by a solid into liquid He there are two separate transmission channels : one which conser-ves the parallel component of phonon wavevector,

and one which does not.It has been suggested that interface states are responsible for the scattering of phonons, thereby creating this latter channel of transmission, and that it is this channel which en-hances the Kapitza conductance above the acoustic mismatch value of the Khalatnikov model /3/. Fur-ther investigation into the nature of these inter-face states is therefore necessary, and this work was concerned with their effect on the reflection

4

of phonons at a liquid He - solid interface. NaF was chosen for the solid as it can be cleaved to form an essentially flat surface on an atomic scale and also because most of the transmission work cited above was carried out with this material.

For this work phonons were reflected from the helium side of the interface as helium is in many ways an ideal transport medium for phonons, being isotropic and supporting only one propagating phonon mode. The experimental arrangement is shown schematically in figure 1. The incident phonon beam in the helium was produced and collimated by a wire heater-(5mm x 15 ym dia., 50 Q resistance) and

col-limating slits, and in this preliminary experiment the angular variation of the scattered phonon

intensity was measured with a graphite film bolome-2

ter (1mm ) .

H E L I U M T = 0 1 K

COLLIMATING

Fig.l.: Schematic diagram of the phonon reflection apparatus, showing a perspective and plan view, and indicating the range of movement of the bolometer.

The angle of grazing incidence was fixed at a = 9.8°. The heater was pulse-heated for 5 (hea-ter temperature = 5K) and the phonons generated in the helium, after traversing the collimating slits, were then either incident on the cleaved (100) face of a NaF crystal or were undeviated and passed abo-and below the crystal. The bolometer could be moved relative to the crystal at a constant radial dis-tance of 12mm from the interaction area formed by the incident phonon beam and the crystal face, and in this way it could monitor the reflected phonon signal as a function of angle. Also around the 8 = 0 position a part of the incident phonon beam + The work described here was carried out during

the course of an ICI Fellowship at Nottingham Uni-versity Physics Dept., Nottingham,England.

could be measured for comparisons of angular with

and amplitude. The heater

-

bolometer flight time

for the phonons was

60ps, and to avoid unwanted

internal reflections within the apparatus measure-

ments were only made on the first 5ys of the detec-

ted pulse.

The experiment was performed in a cell filled

with liquid 4 ~ e

at

K and pressurised to 24bar,

so that phonon free paths in the helium would be

long /4/. Detected pulses were amplified and avera-

ged using a Biomation 8100 transient recorder and

a Nicolet 1072 signal averager. The angle0 was

changed by an in situ stepping motor and gear train

which together produced incremental displacements

of A0

0.6',

and was measured by a potentiometer

coaxial with the rotating part of the apparatus.

-1b

6

ib

e DEGREES.

Fig.2

The angular distribution of phonons reflec-

ted from the cleaved

face of a NaF crystal

immersed in liquid 4 ~ e

at

T

0.1K; The angle of

grazing incidence was

9.8O.

The results are shown in figure 2. The smal-

ler of the two peaks at 0

is due to the direct

incident beam and shows the degree of collimation.

The angular position of the larger peak is'in very

good agreement with

2ci

,

as indicated by the

arrow,and is clearly due to specular reflection of

the phonons at the helium-NaF interface.Note that the

FWHM of this peak is not significantly broader than

that of the incident beam,and is

4

.However ,from

the geometry of the apparatus under rectilinear phpnon

propagation the reflected

signal should be

22

times larger than the direct

0 =

signal.

In conclusion it appears that one component

of the incident phonon flux (by far the larger in

this case) has been strongly scattered at the inter-

face and has been lost from the expected specular

reflection peak, whilst the other component remains

(within the experimental resolution) unaffected.

This situation could arise for example if there are

surface states at the liquid 4 ~ e

-

NaF interface

wich interact strongly with only those phonons abo-

ve some.threshold frequency. The scattered phonons

might then be identified with high frequency pho-

nons above the threshold, and the observed

0

2

peak would consist entirely of lower frequency pho-

nons in this model. It is hoped that these results

may stimulate further work in this area

-

perhaps

using superconducting tunnel junctions as frequency

selective elements.

Finally I would like to thank Pr.A.F.G.Wyatt

for his help and encouragement during the earlier

stages of this work.

References

/ l /

Wyatt,A.F.G., Page,G.J. and Sherlock R.A.,Phys.

Rev.Lett.36 (1976) 1184, and references therein.

/2/ Sherlock,R.A.,Mills ,N.G.,and Wyatt,A.F.G.

J.Phys. C

5

(1975) 300